Weak acidic bright ductile zinc electrolyte

ABSTRACT

An aqueous acidic zinc bath is used for electroplating bright ductile metallic zinc deposits on metal substrates, said plating bath containing at least one soluble zinc compound capable of being plated by electrolytic deposition and a polypropoxy ether or a polypropoxy-ethoxy ether interpolymer, the amount of ethylene oxide moeity does not exceed 25% by weight of the interpolymer.

United States Patent 091 Steeg [451 Dec. 23, I975 l l WEAK ACIDIC BRIGHT DUCTILE ZINC 3,767,540 l0/l973 Rosenberg 204/55 R ELEC'I'ROLY'I'E I w J hi 8 Ge r S FOREIGN PATENTS OR APPLICATIONS g mge) 602,591 S/l948 United Kingdom 204/55 R [73] Asslgnee: rgi gg gga Primary ExaminerG. L. Kaplan Attorney, Agent, or Firm-Pennie & Edmonds [22] Filed: Sept. 11, I974 [2!] App]. No.: 505,065 [57] ABSTRACT 30 Foreign Application p i Dam An aqueous acidic zinc bath is used for electroplating Sap I8 German 2346942 bright ductile metallic zinc deposits on metal suby strates. said plating bath containing at least one soluble zinc compound capable of being plated by electroliil a?SIJJJJJJJJJJJJJJJJJJJ: b52352; We depogfion a or a Poly;

propoxye oxy e er interpo ymer, e amount 0 [58] Field of Search 204/55 R, 43 Z, 44, I I4 ethylene oxide moeity not exceed 25% by weight [56] Ram Chad of the interpolymer.

6/l972 Harbulak 204/55 R WEAK ACIDIC BRIGHT DUCTILE ZINC ELECTROLYTE BACKGROUND OF THE INVENTION The present invention relates to an acid plating bath for depositing metallic zinc on a substrate in which the zinc deposit produced thereby is ductile and presents a glossy to bright coating on the substrate over a wide cathodic current density range.

In recent years there have been numerous processes for the electrolytic deposition of bright zinc coatings from baths within the mild acid range. These baths yield bright coatings within a wide cathodic current density range. and have been used to a great extent in general electroplating. Several properties of these baths and of the zinc coatings deposited therefrom, however. impose limitations on their use.

The ductility of the bright coatings deposited from heretofore known acid zinc baths is distinctly lower than the coatings from alkaline baths. Several patents relating to the deposition of bright zinc coatings from acid baths state that their processes permit the deposi tion of ductile zinc coatings. Measurement of the breaking elongation of zinc layers obtained from such baths, as an index of their ductility, shows differences between the results of the particular processes. but also show that the breaking elongation in all cases is under 1.0%. The breaking elongation is defined as the percent that the length of the electroplated substrate may be increased before the zinc coating thereon develops cracks or fissures. The ductility of these coatings is thus considerably lower than in the case of deposition from alkaline cyanide baths. in which breaking elongations of [.5 to 3.0% are found. This also agrees with practical experience and with the data in the literature, for example D. .lahn. Metalloberflache 24 1970) 7. p. 229ff. This low ductility limits the thickness of the bright zinc coatings deposited from mild acid baths, especially if the plated parts are to be subsequently worked. In the case of coatings of approximately [5 pm thick. cracks or fissures or even scaling off of the coating may occur.

Almost all baths of the prior art used for depositing bright zinc coatings within the mild acid range also contain large quantities of ammonium salts. (In general. more than I00 g/l of ammonium chloride but usually 150 to 200 g/l.) Some baths even contain substances which form stable complexes with zinc and which lead to difficulties in the treatment of the waste water of such baths. Reduction of the zinc content in the waste water of such ammonium salt-containing baths to acceptable limits is possible by means of simple neutralization only if this waste water contains no substances that form stable complexes with zinc within the range of neutralization and if such waste waters are treated singly. If however. as is generally customary in electroplating. the waste waters of all acid baths are combined and then neutralized jointly, there may be formed amine complexes of other heavy metals. such as for example nickel and copper in cases where the content of ammonium salts is too high. These complexes will prevent complete precipitation or require special measures for effecting the precipitation of the metals.

The heretofore known baths for the deposition of bright zinc coatings within the mild acid range also contain rather large quantities of surface active substances. Because of the heavy formation of froth these baths may be operated with air injection for increased production only in exceptional cases. This tendency of HISTORY OF THE PRIOR ART The prior art is exemplified in US. Pat. No. 3.729.394. which discloses a mild acid plating bath containing interpolymers of propylene oxide and ethylene oxide. wherein the ethylene oxide portion amounts to 35 to 45% of the surfaceactive substance. If an additional boundary surface-active substance is used the ethylene oxide portion amounts to 30 to The additional surface-active compounds of the patent include N-(alkylsulfonyl )-glycine and imidazoline derivatives and the baths also contain ammonium salts.

These baths with or without the addition of a boundary surface-active substance show considerable frothing capacity. so that they have all of the above-mentioned disadvantages associated with the formation of froth. In addition, their ammonium salts content may lead to the above described difficulties in waste water treatment. Further. study of the examples of this patent revealed that while bright zinc coatings may be obtained. the breaking elongation. however. is only between 0.6 and 1.3% and thus ranges within the order of magnitude of heretofore known mild acid bright zinc baths.

DESCRIPTION OF THE INVENTION The object of the present invention is to obtain an acid plating bath of the type defined above which is free of the aforesaid disadvantages and in particular makes possible the deposition of bright zinc layers with greater ductility.

This object is achieved with an acid bath characterized by the incorporation of polypropoxy ether and/or polypropoxyethoxy ethers containing up to a maximum of 25% by weight of oxyethylene units based on the total weight of the copolymer.

Surprisingly it has been found that the use of a polypropoxy ether and/or a polypropoxy-ethoxy ether containing a maximum of 25% by weight of oxyethylene units based on the weight of the copolymer results in baths which produce essentially more ductile glossy to bright zinc coatings than baths known heretofore. Further. the breaking elongation of these coatings amounts to up to 3%. and hence lies within the range of coatings deposited with alkaline baths. Thus. the field of application of acid bright zinc baths is extended also to the deposition of thicker coatings. especially if the latter are to be subsequently worked.

Unexpectedly it has been found further that when the polymerization products described above are used pursuant to the invention the use of ammonium salts is not necessary and they may be replaced by simple salts of sodium or potassium. Ammonium salts may. however. continue to be used and. moreover. mixed alkali salts. optionally together with ammonium salts. may also be used. The present invention thus makes it possible to use acid bright zinc baths having none or only a very small portion of ammonium salts. This procedure eliminates the above-mentioned difficulties inherent in the treatment of waste waters of acid bright zinc baths that have a high content of ammonium salts.

Baths pursuant to the present invention have the further advantage of showing no or only very little formation of froth. thus avoiding the possible resultant difficulties in the use of air injection for increased pro- 3 duction as described above.

The polymerization products used pursuant to the present invention vary in their solubility within the bath. Their solubility may, however, be adjusted to meet the bath requirements. In general their solubility decreases with increasing molecular weight and in creases with increasing content of ethylene oxide. Additionally, the polymerization products not sufficiently soluble in the bath may nevertheless be used if they are emulsified by an emulsifier compatible with the bath and in sufficient quantity in the bath.

Baths pursuant to the present invention yield zinc coatings whose ductility is better than that of acid baths known heretofore. They in all cases lie within the limits given above, i.e., a breaking elongation of up to 3%. The most ductile coatings are obtained with baths pursuant to the invention with interpolymers of propylene oxide and ethylene oxide having an ethylene oxide content of to The polypropoxy ethers and the polypropoxy-ethoxy ethers can be used alone or in mixture with each other, and are the polymerization products of propylene oxide and/or propylene oxide and ethylene oxide. These products are well known and can also be prepared in manners well known in the art. See, for example, Schwartz, Perry & Berch, SURFACE ACTIVE AGENTS AND DETERGENTS, Vol. 11, lnterscience (1958), pp. 163-166; MOlLLlET, COLLIE & BLACK, Van Nostrand (1961 pp. 474478; and the ENCYCLOPEDlA OF POLYMER SCIENCE AND TECHNOLOGY, Volv 6 (1967), pp. 103-209. These polymerization products, as is well known in the art, have terminal hydroxyl groups.

The polymerization products, polypropoxy ethers and/or polypropoxy-ethoxy ethers, incorporated in baths pursuant to the invention, are used in quantities of 1 g/l up to the limit of solubility or emulsifiability in the bath, preferably however, in quantities of approximately 3 to 20 g/l. These polymerization products have, in the case. of the polypropoxy ethers, a molecular weight of advantageously between about 300 and 1500, and in the case of the polypropoxy-ethoxy ethers interpolymers a molecular weight of advantageously be' tween about 1000 and 5000. As mentioned above, emulsifiers may be used with the bath composition to colloidally suspend the polymerization products in the bath solution. The emulsifiers used should, of course, be sufficiently soluble in the bath and compatible with the remaining bath constituents and have sufficient emulsifying capacity for the polymerization products used in each case. They may be anionic, non-ionic, amphoteric or cationic. Particularly suitable are anionic boundary surface-active substances such as alkyl sulfates, alkyl sulfonates, alkyl-substituted diphenyl oxide sulfonates and sulfated polyglycol ethers. How ever, nonionic, amphoteric or cationic boundary surface-active substances, if they are compatible with the bath, may also be used. Similarly, emulsifiers such as, for example, condensation products of naphthalene sulfonic acid and formalin are also usable. Preferable are such emulsifiers which themselves have only a relatively small frothing capacity or whose frothing capacity is largely inhibited by the defrothing effect of the polymerization products used pursuant to the invention. The quantity of emulsifier used depends upon the type and quantity of the condensation products used and upon its emulsifying capacity. It is in general be tween about l and 20 g/l.

The preferred zinc compounds include the customarily water-soluble zinc compounds and are employed in the usual quantities well known in the art. Zinc chloride. zinc nitrate, zinc acetate, zinc fluoroborate and zinc sulfate are preferable.

Alkali salts that may be used the simple salts of sodium, potassium or ammonium, include the sulfates, chlorides, fluoroborates, acetates alone or in mixtures. The alkali salts are used in quantities of from about 5 to about 250 g/l, preferably in quantities of about 20 to about 180 g/l.

Pursuant to the invention conventional brightening agents may be included in the bath such as carbonyl compounds which are well known in the art. More specifically, brightening agents which can be used in the baths include aromatic carbonyl compounds, aldehydes, ketones or salts of carboxylic acids. Ketones preferably are used and benzal acetone is a particularly acceptable ketone. Particularly good brightness within a wide cathodic current density range can be obtained in many cases if two or more carbonyl compounds are used simultaneously, of which one is the salt of an aromatic carboxylic acid, such as sodium benzoate. The aldehydes and ketones are used in quantities of between about 0.05 and 3 g/l, the salts of aromatic carboxylic acids in quantities of between about 1 and 20 g/l.

Baths pursuant to the present invention can be used with agitation, preferably mechanically or by air. The electrolytic deposition can be carried out at a pH of between about 3 and 6, preferably from between about 4 and 5.8, and within a temperature range of between about 15 and 30C.

Depending upon the metal content of the baths, the type of article being plated and type and strength of agitation of the bath, cathodic current densities of up to SA/dm may be attained. 1f the baths are operated by air flow, then even considerably higher cathodic current densities are possible.

The bath compositions listed below yield in an aque ous bath at a temperature of 20 to 25C., within wide cathodic current density range, preferably at current densities of 3 A/dm, and pH within a range of 4 to 5.5 with moderate bath agitation and exposure times of 10 to 15 minutes, bright to very bright zinc deposition.

Various articles can, of course, be plated according to this invention as is well known in the art. The zinc plating takes place on the cathode and the majority of zinc plating is done on steel articles inserted into the bath as cathodes. The following examples are illustrative of the invention and as such are not intended to express limitations on the invention as defined within the specification.

Exam 1e 1 g/l Zinc chloride g/l Sodium chloride 12 g/l Dccyloxydiphenyldisulfonic acid; Na salt 14 g/l Polypropoxy ether having an approximate molecular weight of 400 9 g/l Sodium hcnzoate 015 g/l Benzal acetone Example 2 70 g/l Zinc chloride 70 g/l Ammonium chloride 9 g/l Condensation product of naphthalene sulfonic acid and formalin 5 g/l A polyproxyethoxy other with an oxyethylene content of about 25% having an approximate molecular weight 01 3000 4 g/l Sodium bcnzoate continued -continued lixample 2 0.1 g/l Example 3 70 g/l 60 g/l 0.] f Example 4 0.1 g/l Example 5 100 g/l ()5 g/l Example 6 200 g/l 8 g/l 0.l g/l Example 7 75 g/l 80 g1] 20 g/l 9 g/l l2 g/l 5 gll 0.3 g/l Example 8 Example 9 80 g/l 100 g/l 8 all 7 g/l 5 g/l 0.4 g/l Example I0 80 g/l 30 g/l 20 g]! 20 g/l 8 g/l I2 g/l 0.4 g/l Example l l 50 g/l 90 g/l 10 g/l 12 g/l Mcthylnaphthyl kctone Zinc chloride Sodium chloride Ammonium chloride Quaternized. cthoxylized aliphatic amine"- lsononyl alcohol olyglycol ether A polypropoxy-et ox ether having an oxyethylene content with a polyct ylene oxide of about l0lr and having an approximate molecular weight of 2000 Polypropoxy cther having approximate molecular weig t of I200 Sodium bcnzoate Benzal acetone Zinc chloride Ammonium acetate Dodecyloxydiphenyl disulfonic acid; Na salt Polypropoxy-ethox ether having an oxyethylene content of about I ii and having an approximate molecular weight of l050 Benzal acetone Zinc chloride Sodium acetate Ethylhexyl sulfate Polypropoxy-ethoxy ether having an oxyethylene content of about 20% and an approximate molecular weight of 2000 Polypropoxy-ethoxy ether having an oxyethylene content of about [0% and an approximate molecular weight of 2000 Benita] acetone Zinc sulfate 7 H O Ammonium chloride Polypropoxy ether having an approximate molecular weight of 400 Sodium bcnzoate Benzal acetone Zinc chloride Sodium chloride Ammonium fluoroborate Dcclyoxydiphenyl disulfonic acid; Na salt'" Polypropoxy-ethoxy ether having an oxyethylene content of about I0)? and having an approximate molecular weight of 2000 Sodium benzoate 3,5-Dichlorsalicyl aldehyde Zinc chloride Ammonium sulfate Dccycloxydiphenyl disulfonic acid; Na salt Pol propoxy ether havin an approximate mo ecular weight of 12 Sodium benzoate Benzal acetone Zinc chloride Sodium chloride Decyloxydiphenyl disulfonic acid, Na salt'" Polypropoxy-ethox ether having an oxyethylene content of about 1 and an approximate molecular wei ht of 2000 S ium benzoate Bcnzophenone Zinc chloride Ammonium chloride Decyloxydiphenyl disuli'onic acid; Na salt' Polypropoxy ether having an approximate mo ecular weight of 400 Polypropoxy other havin an approximate mo ecular weight of I20 o-Chlorbenzaldehyde Example i2 100 g/l Zinc chloride 5 75 g/l Potassium acetate 6 g/l Sulfated pol glycol other of a natural fatty alcoho 8 g/l Polypropoxy-ethox ether having an oxycthylene content of about 2 it and an approximate molecular weight of 2500 1 g/l Polypropoxy other having an approximate to molecular weight of 1200 0.5 g/l Benzal acetone Example I3 I60 7l Zinc sulfate 7H O 30 g/l Ammonium acetate 6 g/l Condensation product of naphthaline sulfonic acid and formalin 15 l g/l Alkyl amidobetaine" 14 g/l Pol propoxy ether having an approximate mo ecular weight of 400 5 g/l Polypropoxy-ethox ether having an oxyethylene content of about 2 and an approximate molecular weight of 3000 5 g/l Sodium benzoate (H g/l Benzal acetone Example 14 80 g/l Zinc chloride [0 g/l Ammonium chloride 20 g/l Decyloxydiphenyl disulfonic acid; Na salt" 16 g/l Polypropoxy ether having an approximate molecular weig tof 1200 10 g/l Sodium benzoate 0.2 g/l Benzal acetone Example I5 60 g/l Zinc chloride 60 g/l Potassium chloride g/l Ammonium chloride 8 g/l Ouaternized ethoxylized aliphatic amine 30 8 g/l Condensation product of naphthalene sulfonic acid and formalin 8 g/l Polypropoxy-ethoxy ether having an ethylene oxide content of about 20% and an approximate molecular weight of 2000 0.5 g/l Polypropoxy ether having an approximate molecular weight of 1200 0.4 g/l Benzal acetone IDENTITY OF KEYED COMPOUNDS l. corresponds to formula:

SO Na SO Na 2. Oleylamine with 6 to 7 mols ethylene oxide, quaternized with dimethyl sulfate.

3. Non-ionic surface-active compound of isononyl alcohol and approximately l5 mols ethylene oxide.

4. Compound of the formula:

C H,,-iHCH-,OSO Na 5. C, -C ,,-aliphatic alcohol sulfated with mols ethylene oxide.

6. Corresponds to formula:

I claim:

I. An aqueous acid zinc plating bath substantially void of ammonium ions for depositing ductile glossy to bright zinc layers within a wide cathodic current density range containing a water-soluble zinc compound capable of plating zinc metal on a substrate at least about 1 g/l of a polypropoxy ether having a molecular weight of from about 300 to about l500 and/or a polypropoxy-ethoxy ether having a molecular weight of from about 1,000 to about 5,000 and containing up to about 25% by weight of the polymer of the oxyethylene moiety and an aromatic carbonyl compound.

2. An aqueous acid zinc plating bath according to claim 1 in which the polymer is present in an amount from about 1 g/l to its limit of solubility.

3. An aqueous acid zinc plating bath according to claim 2 in which the polymer is present in concentrations of between 1 to 30 g/l.

4. An aqueous acid zinc plating bath according to claim 2 in which the pH is between about 4 and 6.

S. An aqueous acid zinc plating bath according to claim 1 in which the polymer is insoluble in the baths or exceeds the solubility of the polymer in the bath, and in which the polymer is emulsified with an emulsifying agent.

6. An aqueous acid zinc plating bath according to claim 5 in which the emulsifying agent includes alkyl sulfates, alkyl sulfonates, alkyl-substituted diphenyl oxide sulfonates or sulfonated polyglycol esters.

7. An aqueous acid zinc plating bath according to claim 5 in which the surface active emulsifying agents include the sodium salt of decyloxydiphenyldisulfonic acid; the condensation product of naphthalene sulfonic acid and formalin; oleylamine having 6 to 7 mols of ethylene oxide attached thereto and quaternized with dimethyl sulfate; isononyl alcohol having mols of ethylene oxide attached thereto; the sodium salt of dodecyloxydiphenyl disulfonic acid; ethylhexyl sulfate C -C sulfated aliphatic alcohols having from 70 to 90 mols of ethylene oxide attached thereto or dodecane amidobetaine.

8. An aqueous acid zinc plating bath according to claim 1 which contains a mixture of benzal acetone and sodium benzoate a brightening agent.

9. An aqueous acid zinc plating bath for depositing ductile glossy to bright zinc layers within a wide cathodic current density range containing a water-soluble zinc compound capable of plating zinc metal on a substrate and a polypropoxy-ethoxy ether having an approximate molecular weight of between about L000 to about 5 .000 and containing up to about 25% by weight of the polymer of the oxyethylene moiety, said polymer being present in an amount of at least about l g/l.

10. An aqueous acid zinc plating bath according to claim 9 in which the polymer is insoluble in the bath or exceeds the solubility of the polymer in the bath and in which the polymer is emulsified with an emulsifying agent.

H. An aqueous acid zinc plating bath according to claim 10 in which the emulsifying agents include alkyl sulfates, alkyl sulfonates, alkyl-substituted diphenyl oxide sulfonates, or sulfonated polyglyoxal esters.

12. An aqueous acid zinc plating bath according to claim 10 in which the emulsifying agents include the sodium salt of decyloxydiphenyl disulfonic acid; the condensation product of naphthalene sulfonic acid and formalin; oleylamine having 6 to 7 mols of ethylene oxide attached thereto and quaternized with dimethyl sulfate; isononyl alcohol having 15 mols of ethylene oxide attached thereto; the sodium salt of dodecyloxydiphenyl disulfonic acid; ethylhexyl sulfate C C sulfated aliphatic alcohols having from to mols of ethylene oxide attached thereto or dodecane amidobetaine.

13. An aqueous acid zinc plating bath according to claim 9 which includes a brightening agent comprising one or more aromatic carbonyl compounds.

14. An aqueous acid zinc plating bath according to claim 13 in which the brightening agent is a mixture of benzyl acetone and sodium benzoate.

15. An aqueous acid zinc plating bath according to claim 9 in which the polymer is present in concentrations of between about 1 to 30 g/l. i I

9 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,928,149 Dated December 23 1975 Inventofls) Joachim Steeg It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 42, "benzyl" should read benzal Signed and Scaled this Twenty-seventh Day Of July 1976 [SEAL] Arrest.-

RUTHC. Mei SON C. MARSHALL DANN Arresting ()jjicer Commissioner ufPalelzts and Trademarks 

1. AN AQUEOUS ACID ZINC PLATING SUBSTANTILLY VOID OF AMMONIUM IONS FOR DEPOSITING DUCTILE GLOSSY TO BRIGHT ZINC LAYERS WITHIN A WIDE CATHODIC CURRENT DENSITY RANGE CONTAINING A WATER-SOLUBLE ZINC COMPOUND CAPABLE OF PLATING ZINC METAL ON A SUBSTRATE, AT LEAST ABOUT 1 G/L OF A POLYPROPOXY ETHER HAVING A MOLECULAR WEIGHT OF FROM ABOUT 300 TO ABOUT 1500 AND/OR A POLYPROPOXY-ETHOXY ETHER HAVING A MOLECULAR WEIGHT OF FROM ABOUT 1,000 TO ABOUT 5,000 AND CONTAINING UP TO ABOUT 25% BY WEIGHT OF THE POLYMER OF THE OXYETHYLENE MOIETY AND AN AROMATIC CARBONYL COMPOUND.
 2. An aqueous acid zinc plating bath according to claim 1 in which the polymer is present in an amount from about 1 g/l to its limit of solubility.
 3. An aqueous acid zinc plating bath according to claim 2 in which the polymer is present in concentrations of between 1 to 30 g/l.
 4. An aqueous acid zinc plating bath according to claim 2 in which the pH is between about 4 and
 6. 5. An aqueous acid zinc plating bath according to claim 1 in which the polymer is insoluble in the baths or exceeds the solubility of the polymer in the bath, and in which the polymer is emulsified with an emulsifying agent.
 6. An aqueous acid zinc plating bath according to claim 5 in which the emulsifying agent includes alkyl sulfates, alkyl sulfonates, alkyl-substituted diphenyl oxide sulfonates or sulfonated polyglycol esters.
 7. An aqueous acid zinc plating bath according to claim 5 in which the surface active emulsifying agents include the sodium salt of decyloxydiphenyldisulfonic acid; the condensation product of naphthalene sulfonic acid and formalin; oleylamine having 6 to 7 mols of ethylene oxide attached thereto and quaternized with dimethyl sulfate; isononyl alcohol having 15 mols of ethylene oxide attached thereto; the sodium salt of dodecyloxydiphenyl disulfonic acid; ethylhexyl sulfate C16-C18 sulfated aliphatic alcohols having from 70 to 90 mols of ethylene oxide attached thereto or dodecane amidobetaine.
 8. An aqueous acid zinc plating bath according to claim 1 which contains a mixture of benzal acetone and sodium benzoate as a brightening agent.
 9. AN AQUEOUS ACID ZINC PLATING BATH FOR DEPOSITING DUCTILE GLOSSY TO BRIGHT ZINC LAYERS WITHIN A WIDE CATHODIC CURRENT DENSITY RANGE CONTAINING A WATER-SOLUBLE ZINC COMPOUND CAPABLE OF PLATING ZINC METAL ON A SUBSTRATE AND A POLYPROPOXYETHOXY ETHER HAVING AN APPROXIMATE MOLECULAR WEIGHT OF BETWEEN ABOUT 1,000 TO ABOUT 5,000 AND CONTAINING UP TO ABOUT 25% BY WEIGHT OF THE POLYMER OF THE OXYETHYLENE MOIETY, SAID POLYMER BEING PRESENT IN AN AMOUNT OF AT LEAST ABOUT 1 G/L.
 10. An aqueous acid zinc plating bath according to claim 9 in which the polymer is insoluble in the bath or exceeds the solubility of the polymer in the bath and in whicH the polymer is emulsified with an emulsifying agent.
 11. An aqueous acid zinc plating bath according to claim 10 in which the emulsifying agents include alkyl sulfates, alkyl sulfonates, alkyl-substituted diphenyl oxide sulfonates, or sulfonated polyglyoxal esters.
 12. An aqueous acid zinc plating bath according to claim 10 in which the emulsifying agents include the sodium salt of decyloxydiphenyl disulfonic acid; the condensation product of naphthalene sulfonic acid and formalin; oleylamine having 6 to 7 mols of ethylene oxide attached thereto and quaternized with dimethyl sulfate; isononyl alcohol having 15 mols of ethylene oxide attached thereto; the sodium salt of dodecyloxydiphenyl disulfonic acid; ethylhexyl sulfate C16-C18 sulfated aliphatic alcohols having from 70 to 90 mols of ethylene oxide attached thereto or dodecane amidobetaine.
 13. An aqueous acid zinc plating bath according to claim 9 which includes a brightening agent comprising one or more aromatic carbonyl compounds.
 14. An aqueous acid zinc plating bath according to claim 13 in which the brightening agent is a mixture of benzyl acetone and sodium benzoate.
 15. An aqueous acid zinc plating bath according to claim 9 in which the polymer is present in concentrations of between about 1 to 30 g/l. 